Method for providing lateral thermal processing of thin films on low-temperature substrates

ABSTRACT

A method for thermally processing a minimally absorbing thin film in a selective manner is disclosed. Two closely spaced absorbing traces are patterned in thermal contact with the thin film. A pulsed radiant source is used to heat the two absorbing traces, and the thin film is thermally processed via conduction between the two absorbing traces. This method can be utilized to fabricate a thin film transistor (TFT) in which the thin film is a semiconductor and the absorbers are the source and the drain of the TFT.

PRIORITY CLAIM

The present application claims priority under 35 U.S.C. §119(e)(1) toprovisional application No. 61/350,765 filed on Jun. 2, 2010 andapplication number 13/152,065 filed on Jun. 2, 2011, the contents ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a method for curing thin films onsubstrates in general, and, in particular, to a method for thermallyprocessing thin films on low-temperature substrates.

2. Description of Related Art

In general, thermal processing encompasses sintering, annealing, curing,drying, crystallization, polymerization, chemical reaction initiationand modulation, dopant drive-in, degasification, etc. Thermal processingof semiconductor thin films is typically performed in high temperatureenvironments. For example, amorphous silicon (a—Si) is annealed at1,100° C., and silicon nanoparticle films are sintered at 900° C. Thus,the high-temperature requirement for processing semiconductor thin filmsoften mandates the usage of high-temperature substrates, such as firedceramics or quartz, as the choice substrates for carrying semiconductorthin films. Needless to say, it is more desirable to use low-temperaturesubstrates, such as borosilicate or soda lime, as the choice substratesfor carrying semiconductor thin films if possible because of theirrelatively low cost. Even more desirable substrate materials would beplastic (i.e., polycarbonate, polyimide, PET, PEN, etc.) or paperbecause their cost is even lower.

However, the usage of equipment that can provide an equilibrium process,such as an oven, is not a viable option for thermally processing asemiconductor thin film on a low-temperature substrate. This is becausethe required temperature for annealing and sintering most, if not all,semiconductor thin films are considerably higher than the maximumworking temperatures of low-temperature substrates such as polyimide andPET, which are around 450° C. and 150° C., respectively.

The present disclosure provides a method for thermally processing thinfilms on low-temperature substrates.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, twoabsorbing traces spaced apart are in thermal contact with a thin filmlocated on top of a substrate. Pulsed radiation is utilized to heat thetwo absorbing traces, and the heat from the two absorbing traces issubsequently conducted in the plane of the thin film to the thin filmbetween the two absorbing traces to thermally process the thin film.

The above-mentioned process may be used to fabricate a thin filmtransistor (TFT). For example, two absorbing traces, which may becomposed of metal or ceramic, can be used to form a source and drain ofa TFT, and a semiconductor thin film can be used to form an activechannel of the TFT.

All features and advantages of the present invention will becomeapparent in the following detailed written description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention itself, as well as a preferred mode of use, furtherobjects, and advantages thereof, will best be understood by reference tothe following detailed description of an illustrative embodiment whenread in conjunction with the accompanying drawings, wherein:

FIGS. 1 a-1 b depict a method for thermally processing a thin film, inaccordance with one embodiment of the present invention;

FIGS. 2 a-2 b depict a method for thermally processing a thin film, inaccordance with another embodiment of the present invention;

FIGS. 3 a-3 b depict a method for thermally processing a very thin filmon a low-temperature substrate, in accordance with one embodiment of thepresent invention;

FIG. 4 shows a thin film transistor (TFT) manufactured by the methods ofthe present invention;

FIG. 5 shows a Raman spectrum of an e-beam coated amorphous silicon on aborosilicate glass before and after being exposed to pulsed radiation;

FIG. 6 is a graph showing the selectivity of the pulsed radiationlateral thermal processing method of the present invention; and

FIG. 7 is a graph showing drain current versus drain-source voltage forthe TFT from FIG. 4.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

When using a pulsed radiation thermal processing technique to thermallyprocess a thin film on a substrate, the pulsed radiation emitted fromflashlamps, directed plasma arcs (DPAs), lasers, microwaves, inductionheaters or electron beams has the ability to preferentially heat thethin film over its substrate. In addition, because the heat capacity ofthe substrate is much larger than that of the thin film, and the time ofheating is much shorter than the thermal equilibration time of thesubstrate, the substrate can serve as a heat sink to rapidly cool thethin film immediately after thermal processing.

Although pulsed radiation thermal processing allows a thin film to beheated to a much higher temperature than its substrate can normallywithstand at thermal equilibrium, such thermal processing techniquegenerally depends on the ability of a thin film to absorb the radiationthat is used to heat the thin film. Thus, when a thin film is very thinand/or somewhat transparent, it is quite difficult to thermally processthe very thin film directly with the pulsed radiation thermal processingtechnique because the very thin film typically absorbs minimalradiation. Consequently, an improved method is required to thermallyprocess a very thin film.

Referring now to the drawings, and in particular to FIGS. 1 a-1 b, thereare depicted a method for providing pulsed radiation thermal processingon a very thin film, in accordance with one embodiment of the presentinvention. Initially, a very thin film 12 is deposited on a substrate 14via well-known vacuum techniques. Very thin film 12 may also be coatedor printed on substrate 14. Very thin film 12 can be a fully dense filmor a particulate film. The thickness of very thin film 12 is preferablyless than 10 microns. Next, an absorbing trace 11 is deposited on top ofvery thin film 12 to form a thin film stack 10, as shown in FIG. 1 a.Absorbing trace 11 is preferably made of a material that is moreabsorptive of pulsed radiation than very thin film 12. Examples ofabsorbing trace 11 include metals or ceramics.

When thin film stack 10 is transiently irradiated (i.e., via pulsedradiation) by a light source 15, absorbing trace 11 is preferentiallyheated before very thin film 12. Light source 15 can be a flashlamp,directed plasma arc (DPA), laser, microwave generator, induction heateror electron beam. As a result, the area (shaded area) within very thinfilm 12 and substrate 14 located underneath and adjacent to absorbingtrace 11 is thermally processed by the heated absorbing trace 11, asshown in FIG. 1 b. The distance d₁ within very thin film 12 that isthermally processed can be tens of microns.

With reference now to FIGS. 2 a-2 b, there are illustrated a method forproviding pulsed radiation thermal processing on a very thin film, inaccordance with another embodiment of the present invention. Initially,a very thin film 23 is deposited on a substrate 24 via well-known vacuumtechniques. Very thin film 23 can also be coated or printed on substrate24. Very thin film 23 can be a fully dense film or a particulate film.The thickness of very thin film 23 is preferably less than 10 microns.Next, absorbing traces 21, 22 are deposited on very thin film 23 to forma thin film stack 20, as shown in FIG. 2 a. Similar to absorbing trace11 in FIG. 1 a, absorbing traces 21, 22 are preferably made of amaterial that is more absorptive of pulsed radiation than very thin film23. Examples of absorbing traces 21, 22 include metals or ceramics.Although absorbing traces 21, 22 are shown to be formed on top of verythin film 23, absorbing traces 21, 22 can be formed underneath very thinfilm 23 instead.

Upon being exposed to pulsed radiation from a light source 25, absorbingtraces 21, 22 are preferentially heated over very thin film 23. The heatfrom absorbing traces 21, 22 is then conducted to the area of very thinfilm 23 underneath and/or adjacent to absorbing traces 21, 22, as shownin FIG. 2 b. In FIG. 2 b, an area d₂ within very thin film 23 locatedbetween absorbing traces 21, 22 becomes thermally processed. The gapdistance that can be thermally processed between absorbing traces 21 and22 (i.e., area d₂) is generally larger than d₁ from FIG. 1 b since it isthe overlap of the heat being conducted by two absorbing traces 21, 22and is preferably less than 100 microns. Furthermore, since the areawithin very thin film 23 located between absorbing traces 21 and 22 isthermally processed by the overlapping of heat being conducted from twoabsorbing traces 21, 22, very thin film 23 tends to be more uniformlyprocessed than the area of a thin film adjacent to only one absorbingtrace (such as in FIG. 1 b).

Substrate 14 in FIGS. 1 a-1 b and substrate 24 in FIGS. 2 a-2 b arepreferably high-temperature substrates. However, thermal processing ofvery thin films can also be performed on low-temperature substrates(i.e., maximum working temperatures of 150° C. or less) by applying heatspreading films before or after the application of the absorbing traces.Since the thermal conductivity of the heat spreading film is higher thanthat of the low temperature substrate, heat is preferentially conductedin the plane of the very thin film and the heat spreading film insteadof the low temperature substrate after absorbing traces have beenheated. The heat spreading film also acts as a thermal barrier layer toprotect the low temperature substrate. In addition, the preferentialconduction of heat in the plane of the very thin film increases thedistance at which absorbing traces can be placed from each other. As aresult, a lower energy light pulse can be used to process the very thinfilm, thus making the process more gentle on the low-temperaturesubstrate. The heat spreading film is generally thicker than the verythin film and is generally transparent to the light used to heat theabsorbing traces.

Referring now to FIGS. 3 a-3 b, there are illustrated a method forthermally processing a very thin film on a low-temperature substrate, inaccordance with one embodiment of the present invention. Initially, aheat spreading film 35 is deposited on a substrate 34 via well-knownvacuum techniques. Heat spreading film 35 may be coated or printed onsubstrate 34. A very thin film 33 is then deposited on top of heatspreading film 35 via well-known vacuum techniques. Very thin film 33may be coated or printed on heat spreading film 35. Very thin film 33can be a fully dense film or a particulate film. The thickness of verythin film 33 is preferably less than 10 microns. Next, absorbing traces31, 32 are deposited on very thin film 33 to form a thin film stack 30,as shown in FIG. 3 a. Similar to absorbing traces 21, 22 in FIG. 2 a,absorbing traces 31, 32 are preferably made of a material that is moreabsorptive of pulsed radiation than very thin film 33. Examples ofabsorbing traces 31, 32 include metals or ceramics.

Although absorbing traces 31, 32 are shown to be formed on top of verythin film 33, absorbing traces 31, 32 can be formed underneath very thinfilm 33. Although heat spreading film 35 is shown to be formedunderneath very thin film 33, heat spreading film 35 can be formed ontop of very thin film 33 or absorbing traces 31, 32.

Upon being exposed to pulsed radiation from a light source 35, absorbingtraces 31, 32 are preferentially heated over very thin film 33 and heatspreading film 35. The heat from absorbing traces 31, 32 is thenconducted to the area of very thin film 33 and heat spreading film 35underneath and/or adjacent to absorbing traces 31, 32, as shown in FIG.3 b. In FIG. 3 b, an area d₃ within very thin film 33 and heat spreadingfilm 35 located between absorbing traces 31, 32 becomes thermallyprocessed. The gap distance that can be thermally processed betweenabsorbing traces 31 and 32 is preferably less than 100 microns.

There is a host of materials suitable for being heat spreading film 35.For a low-temperature substrate such as PET, those materials may includehigh-temperature polymers (such as polyimide) or inorganic coatings suchas sputtered metal oxides or spin on glass (SOG). For higher-temperaturesubstrates such as polyimide, more suitable materials for heat spreadingfilm 35 include inorganic coatings such as sputtered metal oxides orSOG. It is preferable that heat spreading film 35 be somewhattransparent in order to maintain transparency of the very thin film andstill allow the selective heating to occur. The required thickness ofheat spreading film 35 is a function of its thermal properties, thethickness and thermal properties of the underlying low temperaturesubstrate, the desired processing temperature of very thin film 33, thedimensions and spacing of absorbing traces 31, 32, and the input radiantheating profile.

One approach of applying a heat spreading film to a high-temperaturesubstrate is to first apply a polymeric coating, which has lower thermalconductivity than the high-temperature substrate, to thehigh-temperature substrate followed by the application of a heatspreading film. This practice retards the diffusion of heat into thethermally conductive substrate and allows a very thin film to beprocessed. An alternative to the polymeric coating is to use ahigh-temperature, low-thermal conductivity inorganic film so that it canwithstand a higher temperature during thermal processing.

One method to achieve a high-temperature, low-thermal conductivityinorganic film is to make the inorganic film porous by using a SOG andload it with porous particles. For example, such an inorganic film canbe made by using silica aerogel nanoparticles loaded in a SOG. Theresulting inorganic film appears to have a thermal conductivity of theorder of (or even lower than that of) PET (i.e., 0.24 W/m-° K.). Sincethe aerogel particles have the SOG matrix, the inorganic film is muchmore durable than a typical aerogel film.

The thermal processing of the very thin film can be tuned by varying thepower and length of the pulsed radiation. Multiple pulses can be used aswell as adjusting the pulse repetition frequency. The shape of the pulsecan be changed using pulse width modulation to further adjust theheating profile. When the pulse length is shorter than the thermalequilibration time of the low-temperature substrate, that is,perpendicular to the plane of the low-temperature substrate, a strongerthermal gradient and higher peak temperature can be generated in it,thereby preferentially heating the very thin film adjacent to theabsorbing traces. The temperature in the very thin film is moreintensely processed near the absorbing traces relative to regionsfarther away from the absorbing traces. Furthermore, pulsed radiationallows the peak processing temperature to be greater than the maximumequilibrium working temperature of a substrate. For example, 150 micronthick PET thermally equilibrates across its thickness in about 35 ms.Thus, a stronger thermal processing gradient as well as a higher peaktemperature can be produced without damaging the low-temperaturesubstrate with a 300 μs pulse than with a 10 ms pulse. A 100 ms pulsecan still heat the very thin film located between the absorbing traces,but the peak temperature that can be maintained is very close to itsmaximum equilibrium working temperature of 150° C. In sum, the maximumpeak temperature that can be achieved in the very thin film withoutdamaging the low-temperature substrate of a longer pulse is less thanthat of a short pulse, but the lateral processing length iscorrespondingly longer also. Since the thermal processing of the verythin film is usually Arrhenius in nature, i.e., the thermal processingis generally related to the exponential of the processing temperaturetimes time, a shorter pulse can process the very thin film moreeffectively than a longer pulse without damaging the low-temperaturesubstrate.

The thickness, width, and spacing of absorbing traces as well as thethickness and thermal properties of a very thin film and underlyinglayers also contribute to the heating profile seen by the very thin filmupon being exposed by pulsed radiation.

The method of the present invention can process very thin films that arenot particularly radiation absorbing. This is particularly relevant tothe fabrication of thin film transistors (TFTs) that are very desirablebecause of their low cost and high performance.

Referring now to FIG. 4, there is depicted a TFT 40 manufactured by theabove-mentioned pulsed radiation thermal processing technique. As shown,a thin dielectric layer 44 is placed on top of two absorbing traces 41and 42 that are located adjacent to a very thin film 43. A conductivetrace 45 is located on top of dielectric layer 44 and absorbing traces41 and 42. Absorbing traces 41, 42 are electrically conductive and formthe source and the drain of a TFT, respectively. Conductive trace 45forms the gate of the TFT. The area located between absorbing traces 41and 42 within very thin film 43 that has been thermally processed is asemiconductor forms the active channel of the TFT. As shown in FIG. 4,the cured area (shaded area) includes the gate oxide and the gate.However, both the gate oxide and the gate are applied after the curingof the very thin film 43.

Very thin film 43 is cured primarily between absorbing traces 41 and 42.Thus, sources and drains can be patterned (or printed) on a very largearea, and very thin film 43 can even be coated over an entire substrate46. Since a cured semiconductor generally has a higher conductivity thanan uncured one, the fact that the semiconductor becomes cured primarilyin the channel of the TFT, the parasitic capacitance of thesemiconductor is reduced. The reduced need for registration and criticaldimensions means that the above-mentioned TFT can be completely printeden mass.

An example of a method for making a TFT, such as TFT 40, is described asfollows. When making a TFT, microcrystalline silicon (μx—Si) is moredesirable as a semiconductor than amorphous silicon (a—Si) because μx—Sihas higher mobility and therefore enables a faster switching TFT. It isusually easier to deposit a—Si followed by a thermal anneal to converta—Si to μx—Si than to deposit μx—Si directly. For example, a 200 nm filmof a—Si on a 500 μm borosilicate wafer can be converted to μx—Si (withan N2 purge) by using a light pulse from a PulseForge® 3300 system(manufactured by NovaCentrix in Austin, Texas) at a threshold voltage of650 V and a pulse length of 100 μs. The light pulse has an intensity ofabout 35 kW/cm², which corresponds to a radiant exposure of about 3.5J/cm².

With reference now to FIG. 5, there is illustrated a Raman spectrum of200 nm a—Si film that was e-beam sputtered coated on a borosilicateglass before and after being exposed to the above-mentioned light pulse.The a—Si film is annealed by the light pulse and is converted to μx—Si.The light pulse is needed to overcome the fact that a 200 nm a—Sicoating only absorbs a portion of the emitted light. An identicalborosilicate wafer is patterned with gold contact source/drain lines toform an eventual TFT of various widths (5-50 μm) and separations (5-50μm). All traces are 5 mm long. The gold patterning is followed by anidentical broadcast electron beam sputtered deposition of 200 nm of a—Sidescribed above over the borosilicate wafer. The borosilicate wafer isthen processed via the above-mentioned PulseForge® 3300 system at a muchlower voltage (i.e., 550 V for 250 μs). The radiant power was 24 kW/cm²,and the radiant exposure was 5.9 J/cm². Note that this level of power isbelow the threshold intensity described above for converting a—Si toμx—Si. Since gold is very absorbing of the light pulse, more energy isabsorbed at those locations.

Referring now to FIG. 6, there is illustrated the selectivity of thepulsed radiation thermal processing method of the present invention. Thegraph shows a comparison of the Ramen spectrum of the thin silicon filmbetween two different gold line pair widths (50 μm and 20 μm) andidentical spacing (50 μm) between the gold traces. The graph shows thatthe space between the 50 μm traces has been converted to μx—Si, whereasthe space between the 20 μm wide traces is unconverted. Similarly, thesilicon film on the rest of the wafer is unconverted. This technique hasconverted the a—Si to μx—Si only between the gold patterned traces andnowhere else achieving automatic registration.

After selective conversion of a—Si to μx—Si between the absorbing traceshas been achieved, a TFT device can be fabricated using a spin-onbarium-strontium-titanate (BST) ceramic as the dielectric layer. Thisdielectric material has a relatively high dielectric constant k (˜300),which allows a high electric field to be imparted to the field-effectchannel of the TFT at low gate voltage. A silver gate metal is vacuumdeposited onto the BST gate dielectric layer to complete the TFT.

Electrical testings can be performed on the TFT to determine if thedrain current can be enhanced by applying a positive gate voltage. Sinceμx—Si is slightly n-type, a positive gate voltage should enhance theelectron concentration in the channel and result in an increased draincurrent (I_(d)).

With reference now to FIG. 7, there is illustrated a graph showing draincurrent (I_(d)) versus drain-source voltage (V_(ds)) for TFT 40 fromFIG. 4. Note that at positive gate voltage (V_(g)), the drain current(I_(d)) is enhanced and has the saturation shape one expects for afield-effect TFT. The linear I-V characteristic observed at negativegate voltage indicates TFT 40 is behaving as a regular resistor when thenegative gate voltage is applied. Reasons for this are unknown at thistime but may be due to hole injection from the source and draincontacts. This effect, if present, is normally reduced/eliminated bysuitably doping the contact regions in order to “block” hole injection.

In summary, using pulsed light annealing of an a—Si thin film withlaterally positioned metal source-drain contacts can be “sub-threshold”annealed to a microcrystalline state within the region between thesource-drain contacts. This has great benefit for the microelectronicsindustry since micro (and nano)-crystalline silicon films have highcarrier mobility and other desirable features that enhance theperformance of thin film devices. Furthermore, since one can convertonly the a—Si in the region between source/drain contacts, leaving thesurrounding regions of the a—Si to remain in a high-resistance amorphousstate, and thus not require patterning or otherwise isolation to limitsuch deleterious effects as parasitic capacitances which limit devicespeed and increase power dissipation.

As has been described, the present invention provides a method forthermally processing thin films on low-temperature substrates. Themethod of the present invention also enables a TFT to be manufactured ina top gate configuration (i.e., gate on top) with minimal registration.Two absorbing traces form a source and drain of a TFT. Before theapplication of the gate oxide and the gate, the thin film material ispreferentially thermally processed between the two absorbing traces. Themethod of the present invention has the effect of selectively curing thethin film material without the need to precisely deposit the material inthe channel of the TFT.

While the invention has been particularly shown and described withreference to a preferred embodiment, it will be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method for thermally processing a very thinfilm, said method comprising: patterning two absorbing traces adjacentto a very thin film, wherein said two absorbing traces are made ofmetal, wherein said thin film is located on top of a substrate;irradiating said two absorbing traces with at least one electromagneticpulse to heat up said two absorbing traces; and allowing heat from saidtwo absorbing traces to thermally process said very thin film.
 2. Themethod of claim 1, wherein said substrate has a maximum workingtemperature of less than 450° C.
 3. The method of claim 1, wherein saidtwo absorbing traces are made of materials more absorptive of saidelectromagnetic pulse than said very thin film.
 4. The method of claim1, wherein said method further includes providing a heat spreading layeradjacent to said very thin film.
 5. The method of claim 4, wherein saidmethod further includes providing a high-temperature, low-thermalconductivity film between said heat spreading layer and said substrate.6. The method of claim 1, wherein said electromagnetic pulse is providedby a flashlamp.
 7. The method of claim 1, wherein said electromagneticpulse is provided by a directed plasma arc.
 8. A method for fabricatinga thin film transistor, said method comprising: patterning two absorbingtraces adjacent to a very thin film, wherein said two absorbing tracesare made of metal, wherein said thin film is located on top of asubstrate; irradiating said two absorbing traces with at least oneelectromagnetic pulse to heat up said two absorbing traces, and allowingheat from said two absorbing traces to thermally process said very thinfilm; depositing a dielectric layer on said two absorbing trace and saidvery thin film; and forming a gate by depositing a conductive trace ontop of said dielectric layer.
 9. The method of claim 8, wherein saidsubstrate has a maximum working temperature of less than 450° C.
 10. Themethod of claim 8, wherein said two absorbing traces are made ofmaterials more absorptive of said electromagnetic pulse than said verythin film.
 11. The method of claim 8, wherein said method furtherincludes providing a heat spreading layer adjacent to said very thinfilm.
 12. The method of claim 11, wherein said method further includesproviding a high-temperature, low-thermal conductivity film between saidheat spreading layer and said substrate.
 13. The method of claim 8,wherein said electromagnetic pulse is provided by a flashlamp.
 14. Themethod of claim 8, wherein said electromagnetic pulse is provided by adirected plasma arc.